Owing to the increasing amount of communications traffic resulting from the increasing number of communications subscribers, and from the increasing requirements for the amount of data to be transmitted, transmitting devices, in particular private branch exchanges, are subject to increasingly severe requirements in terms of the amount of data to be transmitted by a communications link and the number of communications terminals which can be connected to one another.
Present-day devices are based, for example, on the TDM method (Time Division Multiplexing) in which communications data from different connections is transmitted in respectively defined time slots. A connection between different communications partners is produced by a switching matrix which, on the basis of control information, associates incoming time slots on an incoming connection with outgoing time slots on an outgoing connection. Such switching matrices generally have a fixed size and can produce only a defined number of connections, which often makes it harder to adapt switching systems to meet the requirements. Furthermore, the time slots have a restricted capability to accommodate data. For example, one switching matrix can produce 4096 connections, while a maximum of 64 kbits of data can be transmitted within one time slot.
An increase in the number of subscribers can thus be taken into account only in steps of 4096 connections. However, additional development efforts for matching the switching device to the greater number of subscribers is required in order to utilize these different extension levels. It is likewise impossible directly and flexibly to increase the transmission performance of each connection, and this can at best be done by setting up two or more communications links, that is to say in the form of steps of 64 kbits. However, this type of bandwidth multiplication has not been used in practice for transmission via ISDN (Integrated Services Digital Network) and scarcely any terminals are available for this purpose.
Although such existing communications systems have disadvantages which are intended to be overcome in the course of new developments for future communications devices, seamless integration of existing and new communications devices to form a homogeneous communications infrastructure is absolutely essential, at least during a transitional phase.
This is firstly because the investments that have already been made by the end users and communications network operators must be protected. Secondly, this is because it is technically essential to avoid individual communication islands, which the different systems would result in.